Logarithmic amplifiers are commonly used in instruments which receive signals tending to vary over a wide dynamic range. The logarithmic amplifiers, in effect, compress the dynamic range of the input signals, producing output signals whose magnitudes are logarithmically related to the magnitudes of the input signals. Thus input signals varying over an 80 dB range may be compressed, for example, to signals varying over a 20 dB range. The compressed signals may then be applied to signal processing circuitry which processes and analyses the signals without saturating or "jamming".
One technique used to produce a logarithmic output signal is commonly referred to as successive detection. Basically, a series of cascaded amplifiers are connected such that a signal applied to the first amplifier, V.sub.in dB, is amplified and then applied to a second amplifier in the series. The second amplifier amplifies the signal and applies it to a third amplifier in the series, and so on. The output of each amplifier is also applied to a corresponding detector through either a coupler or power splitter. The detector rectifies signals that exceed a predetermined detector threshold voltage, producing an output voltage which is proportional to the applied signal. As V.sub.in dB increases, the cascaded amplifiers successively saturate, that is, the last amplifier in the series saturates first, then the next to last amplifier saturates, and so on. When an amplifier saturates, it is producing a maximum output signal. Thus the corresponding detector is also producing a maximum output signal.
The amplifier/detector pairs are arranged such that when an amplifier produces a signal that saturates the succeeding amplifier, the signal also exceeds the corresponding detector threshold. The detectors associated with the saturated stages are thus producing maximum output signals and the detector preceding the saturated stages is producing an output signal which is proportional to the input signal. The detector output signals are summed to form a video output signal. The output signal, which is piece-wise linear, is approximately logarithmically related to V.sub.in dB.
The operation of each corresponding amplifier and detector must be matched such that when an input signal saturates an amplifier, it also exceeds the threshold of the preceding detector. Thus the detectors must be properly tuned to avoid gaps in the video output signal. The individual stages must be well matched, also, in order to operate properly over a large dynamic range and a wide bandwidth. The amplifiers and detectors are temperature sensitive, and thus matching the stages generally requires two temperature compensation schemes. First, each amplifier and corresponding detector in each stage must be separately temperature compensated, and second, each stage must be temperature compensated to ensure that all of the stages produce output signals which are related V.sub.in dB.
The stages are relatively complex and costly, including several components, namely, an amplifier and corresponding temperature compensating components, a detector and corresponding temperature compensating components, and a coupler or a power splitter. The stages also require special tuning and temperature matching, adding to their cost.
The cost of the stages typically limits the number of stages used to form an amplifier. Thus the video output signal produced by summing the detector output signals is a rough approximation of a signal which is logarithmically proportional to the input signal, V.sub.in dB. Additional video circuitry may be added to the multi-stage amplifier to smooth the output signal into a closer approximation of a signal logarithmically related to the input signal. However, such circuitry further increases the cost.